A satellite-based positioning system or GNSS, comprises a plurality of signal emitters arranged on as many satellites forming a constellation. A minimum of four positioning satellites enable a mobile receiver that can process the signals received from them, to deliver position data for the receiver, in terms of geographical coordinates (x,y,z) at a determined instant t. The signals transmitted by the positioning satellites occupy a wider bandwidth than that required by the throughput of data to be transmitted, with the aim of reducing the influence of the interfering signals, and of reducing the spectral power density of the signals transmitted in such a way that the latter are masked in the background noise. Thus, according to techniques which are in themselves known, the spectrum of the transmitted signals is spread, a carrier wave being modulated by a data signal overlaid on a pseudo-random noise spreading signal with high frequency, according to a periodic sequence specific to each satellite.
According to these techniques, the satellite-based positioning, determined at the level of the receiver, consists firstly in detecting, in an acquisition step, the pseudo-random spreading codes modulating the signals originating from the satellites. Each signal emitted by a visible satellite and received by the antenna of the receiver must then be demodulated by the receiver, so as to determine notably a propagation time measurement and Doppler measurement.
Calculation means implemented in the receiver then make it possible to synchronize, on the satellite signals received, locally generated replicas of these signals. Slaving is undertaken by a carrier loop, steering the phase of the local carrier, and by a code loop steering the position, or phase, of the local code. This synchronization enables the receiver to evaluate the propagation times of the signals originating from the various satellites, and to deduce therefrom its position, by also taking into consideration navigation data contained in the signals.
An acquisition phase typically makes it possible to initialize the operation of the tracking loops, since neither the position, nor the Doppler frequency nor the code received are known a priori; but the tracking loops can operate only if the position of the code and the Doppler frequency are close to those of the useful signal of the satellite considered. If one of the differences is too high, then a zero correlation no longer provides any information, and slaving may not be achieved. To carry out the acquisition phase, a search for a correlation peak is performed between a local signal and the signal received, in a two-dimensional space, by trying a plurality of assumptions regarding the phase of the code and the value of the Doppler frequency, with a sufficiently fine interval to allow detection of the correlation peak. As soon as a correlation peak has been found, the search for the code and for the Doppler frequency may be refined by decreasing the search interval around the detected correlation peak. When the precision obtained is considered to be sufficient, the loops are closed, and converge by construction to the correlation maximum: a so-called “tracking” phase is then entered.
A major cause of positioning errors is related to the presence of multiple paths or “multi-paths” on the signals emitted by the satellites. This phenomenon is related to the reflection of the waves off obstacles, for example buildings, the signal received then being a composite signal consisting of direct signals and reflected signals.
With the aim of reducing multi-path errors, it is possible to resort, according to a technique in itself known, to a so-called “Double Delta” correlator. This solution is presented in the French patent published under the reference FR 2739695, and is described in detail hereinafter. However, this solution presents a risk of false lock-ons, leading to significant or indeed unacceptable positioning errors, up to an order-of-magnitude of a few tens of meters.
A risk of false lock-on is due to the existence of blind zones on the Double Delta code discriminator used for the code loop. If the code error falls within a blind zone in which the code discriminator is zero, the code loop switches to open loop, thereby leading to a static measurement error in the pseudo-range.
Such a risk also exists with other code discriminators exhibiting blind zones.